Partial Substitution of Soybean Meal with a Yeast-Fermented Vinasse Protein Source: Effects on Milk Yield, Composition, and Serum Biochemistry in Crossbred Dairy Cows

Background/Objectives: This study aimed to compare the effects of two distinct dietary protein sources, soybean meal (SBM, 454 g CP/kg dry matter [DM]) and fermented protein source (FPS, 635 g CP/kg DM), on milk yield (MY), milk quality traits, and serum biochemistry in crossbred dairy cows. Methods: This study employed a randomized complete block design with four groups (n = 6 per group): Control (120 g/kg SBM), FPS30 (30 g/kg FPS and 75 g/kg SBM), FPS40 (40 g/kg FPS and 60 g/kg SBM), and FPS50 (50 g/kg FPS and 45 g/kg SBM). Results: After 66 days (21-day adaptation period and 45-day feeding trial), DM intake, milk protein content, and milk density did not differ significantly across groups. However, MYs were significantly higher in the FPS50 group (p = 0.043). Additionally, milk fat (p = 0.021), DM (p = 0.013), solids-non-fat (SNF, p = 0.047), and lactose content (p = 0.007) were significantly higher in the FPS groups. Although serum biochemistry did not differ across groups on day 0, blood urea nitrogen (BUN, p = 0.044) and serum Ca (p = 0.041), P (p = 0.039), and Mg (p = 0.041) concentrations were significantly lower in the FPS groups on day 66. Moreover, FPS intake correlated positively with MY (R = 0.680), milk DM (R = 0.706), fat (R = 0.665), SNF (R = 0.566), lactose content (R = 0.653), and fat-to-protein ratio (R = 0.507) and negatively with milk density (R = −0.429) and mineral content (R = −0.271). Conclusions: These findings indicate that FPS is effective in increasing productivity and improving/maintaining milk quality traits in cows fed diets substituting specific amounts of SBM with FPS. The induced reduction in BUN can be interpreted as increased protein utilization.